Semiconductor light reception element

The present disclosure relates to a semiconductor light receiving element.

Patent Literature 1 describes a photodiode. This photodiode includes a slope reflecting portion formed on an InP substrate, a p electrode, a diffraction lattice, a light reception portion including an InGaAs light absorption layer, and an n electrode. Light incident perpendicularly from a surface is totally reflected by the slope reflecting portion, an optical path thereof is changed to an obliquely upward direction, and the light is incident on the light absorption layer in the light reception portion from obliquely downward. The obliquely incident light propagates in the light absorption layer, is reflected in a reverse direction of the incidence direction by the diffraction lattice and the p electrode provided on the light reception portion, and is absorbed by the light absorption layer again.

In the aforementioned technical field, there is demand for further increase in an operating speed. For this purpose, it is conceivable that a moving distance of electrons be decreased by decreasing the thickness of the light absorption layer. However, a decrease in sensitivity is caused when the thickness of the light absorption layer is decreased. On the other hand, in the photodiode described in Patent Literature 1, the effective thickness of the light absorption layer is increased by causing light to propagate along an optical path in an oblique direction with respect to the light absorption layer. Accordingly, it is considered that the decrease in sensitivity due to the decrease in thickness of the light absorption layer is curbed and an increase in speed is achieved.

However, in the photodiode described in Patent Literature 1, there is concern about an increase in cost because machining for forming the slope reflecting portion on the substrate needs to be performed to form an oblique optical path with respect to the light absorption layer, or the like.

An objective of the present disclosure is to provide a semiconductor light receiving element that can curb an increase in cost and achieve an increase in speed.

According to an aspect of the present disclosure, there is provided a semiconductor light receiving element that receives incidence of light in at least one wavelength band of a 1.3 μm band, a 1.55 μm band, and a 1.6 μm band and generates an electrical signal according to incident light, the semiconductor light receiving element including: a substrate; a semiconductor stacked portion that is formed on a first region of the substrate; and a first electrode and a second electrode that are electrically connected to the semiconductor stacked portion, wherein the semiconductor stacked portion includes: a light absorption layer of a first conductivity type including InGaAs; a buffer layer of the first conductivity type disposed between the substrate and the light absorption layer; and a second region of a second conductivity type other than the first conductivity type that is located on the opposite side to the substrate with respect to the light absorption layer and bonded to the light absorption layer, wherein the first electrode is connected to a first portion of the first conductivity type located on the substrate side with respect to the light absorption layer in the semiconductor stacked portion, wherein the second electrode is connected to a second portion of the second conductivity type located on the opposite side to the substrate with respect to the light absorption layer in the semiconductor stacked portion, wherein an In content x in the light absorption layer is equal to or greater than 0.55, and wherein a thickness of the light absorption layer is equal to or less than 1.8 μm.

The semiconductor light receiving element handles light in wavelength bands for optical communication such as the 1.3 μm band (O-band (Original-band)), 1.55 μm band (C-band (Conventional-band)), and 1.6 μm band (L-band (Long-wavelength-band)). In the semiconductor light receiving element, the light absorption layer provided on the substrate includes InGaAs. The In content x in the light absorption layer is equal to or greater than 0.55 (and less than 1). In this way, when the In content x of InGaAs in the light absorption layer is equal to or greater than 0.55, an absorption coefficient is improved, for example, in comparison with a case in which the In content x is 0.53 (for example, the absorption coefficient is improved two times by setting the content x to 0.62 in the 1.55 μm band). Accordingly, it is possible to avoid a decrease in sensitivity even when the thickness of the light absorption layer is decreased to 1.8 μm or less. That is, it is possible to achieve an increase in speed. In the semiconductor light receiving element, a particular element (for example, the slope reflecting portion of the photodiode described in Patent Literature 1) does not need to be formed at the time of achievement of an increase in speed. Accordingly, with the semiconductor light receiving element, it is possible to curb an increase in cost and to achieve an increase in speed.

In the semiconductor light receiving element according to the present disclosure, the buffer layer may include a strain relaxation layer having a lattice constant between a lattice constant of the substrate and a lattice constant of the light absorption layer. In this case, it is possible to improve crystallinity of the semiconductor stacked portion.

In the semiconductor light receiving element according to the present disclosure, the buffer layer may include a plurality of strain relaxation layers that are disposed such that the lattice constant become close to the lattice constant of the light absorption layer in a stepwise manner from the substrate to the light absorption layer. Alternatively, the buffer layer may include the strain relaxation layer in which the lattice constant changes continuously to become close to the lattice constant of the light absorption layer from the substrate to the light absorption layer. In this case, it is possible to reliably improve crystallinity of the semiconductor stacked portion.

In the semiconductor light receiving element according to the present disclosure, the semiconductor stacked portion may include: a cap layer of the first conductivity type that is provided on the light absorption layer on the opposite side to the substrate with respect to the light absorption layer and includes InAsP; and a contact layer of the first conductivity type that is provided on the cap layer on the opposite side to the substrate with respect to the light absorption layer and includes InGaAs. The second region may be formed from the contact layer to the light absorption layer via the cap layer, and the second portion to which the second electrode is connected may be a surface of the second region formed in the contact layer. In this case, it is possible to decrease contact resistance of the second electrode.

In the semiconductor light receiving element according to the present disclosure, the semiconductor stacked portion may include: a first semiconductor layer of the first conductivity type disposed between the substrate and the light absorption layer; and a second semiconductor layer of the first conductivity type including an impurity concentration lower than an impurity concentration of the first semiconductor layer and disposed between the first semiconductor layer and the light absorption layer. In this case, it is possible to achieve a further increase in speed through a decrease in capacitance.

In the semiconductor light receiving element according to the present disclosure, the semiconductor stacked portion may include a third semiconductor layer disposed between the light absorption layer and the cap layer and having a band gap between a band gap of the light absorption layer and a band gap of the cap layer. In this case, it is possible to curb difficulty in taking out carriers which is due to rapidly changing the band gap between the cap layer and the light absorption layer.

In the semiconductor light receiving element according to the present disclosure, at least one layer of the buffer layer may be semi-insulated by being doped with Fe. In this case, it is possible to improve crystallinity.

In the semiconductor light receiving element according to the present disclosure, the In content x in the light absorption layer may be equal to or greater than 0.57, and the thickness of the light absorption layer may be equal to or less than 1.2 μm. In addition, the In content x in the light absorption layer may be equal to or greater than 0.59, and the thickness of the light absorption layer may be equal to or less than 0.7 μm. In this case, it is possible to achieve an increase in speed due to a further decrease in thickness of the light absorption layer.

In the semiconductor light receiving element according to the present disclosure, the substrate may include a semi-insulated semiconductor. In this case, it is possible to achieve a decrease in capacitance.

In the semiconductor light receiving element according to the present disclosure, the substrate may include an insulator or a semi-insulated semiconductor, and the semiconductor stacked portion may be directly bonded to the substrate. In this way, by separately forming the substrate and the semiconductor stacked portion and directly bonding them to construct the semiconductor light receiving element, it is possible to achieve an increase in diameter and to curb an increase in cost by forming optical components out of cheap materials.

According to the present disclosure, it is possible to provide a semiconductor light receiving element that can curb an increase in cost and achieve an increase in speed.

Hereinafter, an embodiment will be described in detail with reference to the accompanying drawings. In the drawings, the same or corresponding elements will be referred to by the same reference signs, and repeated description thereof may be omitted.

is a schematic side view illustrating an optical device according to an embodiment. As illustrated in, an optical device A includes a semiconductor light receiving element. The optical device A handles light in wavelength bands for optical communication such as a 1.3 μm band (O-band (Original-band)), a 1.55 μm band (C-band (Conventional-band)), and a 1.6 μm band (L-band (Long-wavelength-band)), converts the light to an electrical signal, and outputs the electrical signal. The 1.3 μm band is, for example, a wavelength range of 1.26 μm to 1.36 μm. The 1.55 μm band is, for example, a wavelength range of 1.53 μm to 1.565 μm. The 1.6 μm band is, for example, a wavelength range which is greater than 1.565 μm and equal to or less than 1.625 μm. The light in wavelength bands for communication is light having a peak in one wavelength band of the aforementioned wavelength bands (that is, a wavelength other than the peak may be located outside of the wavelength range of the wavelength bands).

Accordingly, the semiconductor light receiving elementalso handles the light in the wavelength bands and serves to receive incidence of light L of a wavelength belonging to at least one wavelength band of the aforementioned wavelength bands and to generate an electrical signal according to the incident light. The semiconductor light receiving elementis mounted on a sub-mount A. The light L is guided by an optical fiber Aand is condensed on a light reception portion of the semiconductor light receiving elementby a lens A.

The electrical signal generated by the semiconductor light receiving elementis input to a trans-impedance amplifier (TIA) Avia an electrode pad (which is schematically hatched inor the like) provided on the sub-mount Aand a wire, is converted to a voltage by the trans-impedance amplifier A, and is output to the outside. Here, the semiconductor light receiving elementis mounted on the sub-mount Ain a state in which a rear surfaceof a substratewhich will be described later faces the lens Aand the optical fiber A. That is, the semiconductor light receiving elementis used as a rear surface incidence type.

is a plan view of the semiconductor light receiving element illustrated in.is a sectional view taken along line III-III in. As illustrated in, the semiconductor light receiving elementincludes a substrate, a semiconductor stacked portion, a first electrode(a cathode herein), and a second electrode(an anode herein).

The substrateincludes a semi-insulated semiconductor. Here, the substrateis, for example, a semi-insulated semiconductor substrate formed of InP. The substrateincludes a front surfaceand a rear surfaceopposite to the front surface. The substrateincludes a region RA, a region RB (a first region), and a region RC which are sequentially arranged along the front surfaceand the rear surface. The region RB is a region between the region RA and the region RC and is a region on which the semiconductor stacked portion is provided. More specifically, the region RB includes a central region RBand regions RBlocated on two sides (the regions RA and RC sides) of the region RB. Here, the rear surfaceof the substrateis an incidence surface of light L, and a lens RL for condensing light L is formed thereon. The lens RL is formed centered on the region RBto overlap partially the regions RB.

The semiconductor stacked portionis formed on the region RB of the substrateas described above and is a semiconductor mesa protruding from the front surface. The semiconductor stacked portionincludes a buffer layerof a first conductivity type (an N type herein, for example, Ntype). The buffer layeris provided centered on the region RBto overlap partially the regions RB. Here, the semiconductor stacked portionis in contact with the front surfaceof the substratein the buffer layer.

Layers of the semiconductor stacked portionother than the buffer layerare provided in a part overlapping the region RBin the buffer layerin a view seen in a direction crossing the front surface. The buffer layerincludes a first portionexposed from another layer (and a passivation filmwhich will be described later) of the semiconductor stacked portionin a view seen in a direction crossing the front surface, and a junction with the first electrodeis formed in the first portion. The buffer layerincludes, for example, InP. For example, the buffer layeris formed of N—InP.

The semiconductor stacked portionincludes buffer layers,, and, a light absorption layer, a cap layer, and a contact layerwhich are stacked on the buffer layersequentially from the substrateside. The buffer layersandhave a first conductivity type (for example, an Ntype). The buffer layerhas the first conductivity type (for example, an Ntype). The buffer layers,, andinclude InAsP. For example, the buffer layeris formed of N—InAsP, the buffer layeris formed of N—InAsP, and the buffer layeris formed of N—InAsP (or N—InGaAsP).

Accordingly, the buffer layers,, andserve as strain relaxation layers having a lattice constant between a lattice constant of the substrateand a lattice constant of the light absorption layer. That is, the semiconductor stacked portionincludes a plurality of strain relaxation layers (step layers) which are disposed such that the lattice constant becomes close to the lattice constant of the light absorption layerin a stepwise manner from the substrateto the light absorption layer.

The buffer layeris disposed closer to the light absorption layerthan the buffer layersandand has an impurity concentration lower than an impurity concentration of the buffer layersand. Accordingly, the semiconductor stacked portionincludes a first semiconductor layer (the buffer layeror the buffer layer) disposed between the substrateand the light absorption layerand a second semiconductor layer (the buffer layer) having an impurity concentration lower than an impurity concentration of the first semiconductor layer and disposed between the first semiconductor layer and the light absorption layer.

The light absorption layerhas the first conductivity type (for example, an Ntype). The light absorption layerincludes InGaAs. Here, the light absorption layeris formed of N—InGaAs. An In content x of the light absorption layeris equal to or greater than 0.55 (and less than 1). For example, the In content x is 0.59 herein. A thickness of the light absorption layer(a thickness in a stacking direction of the semiconductor stacked portion) is equal to or less than 1.8 μm and is, for example, 0.7 μm herein. The light absorption layermay be an absorption layer of mixed crystals of Al, P, Sb, N, and other materials and InGaAs in a band gap range equal to or less than eV. Proportions of Al, P, Sb, and N (or other materials) which are mixed into InGaAs may be set, for example, to 5% or less or 10% or less.

The cap layerhas the first conductivity type (for example, an Ntype). The cap layerincludes InGaP. For example, the cap layeris formed of N—InAsP herein. The contact layerhas the first conductivity type (for example, an Ntype). The contact layerincludes InGaAs. For example, the contact layeris formed of N—InGaAs herein.

A semiconductor region (a second region) 27 of a second conductivity type (a Ptype herein) is formed in the semiconductor stacked portion. The semiconductor regioncan be formed, for example, by impurity diffusion or ion implantation. The semiconductor regionextends from a top surfaceof the semiconductor stacked portionto the substrate. Here, the top surface(a surface opposite to the substrateside) of the semiconductor stacked portionis a surface of the contact layer. The Ptype semiconductor regionis formed to extend from the contact layerto the light absorption layervia the cap layer.

Here, the semiconductor regionis also formed in the light absorption layer. In an example in which the thickness of the light absorption layeris 0.7 μm, an area of about 0.2 μm on the cap layerside in the light absorption layeris the semiconductor region. That is, in this example, an Nregion with a thickness of 0.5 μm and a Pregion with a thickness of 0.2 μm are included in the light absorption layer, and a boundary therebetween is formed therein. A termination of the Pregion is, for example, a position at which the P-type impurity concentration is equal to or less than 1×10/cm. The boundary between the Nregion and the Pregion may be formed outside of the light absorption layer.

In the aforementioned example, the Ntype means that the N-type impurity concentration is equal to or greater than about 1×10/cm. The Ntype means that the N-type impurity concentration is equal to or lower than about 8×10/cmand is lower than that of the Ntype. The Ptype means that the P-type impurity concentration is equal to or greater than about 1×10/cm.

Here, the semiconductor light receiving elementincludes a passivation film. The passivation filmis, for example, an insulating film. A part of the top surfaceof the semiconductor stacked portionand a side surfaceof the semiconductor stacked portionextending from a circumferential edge of the top surfaceto the substrateside are covered with the passivation film. On the other hand, the remaining part of the top surfaceof the semiconductor stacked portion, that is, the surface of the P-type semiconductor region, is exposed from the passivation film. The second electrodeis formed in the part of the top surfaceexposed from the passivation film, and a junction between the second electrodeand the semiconductor region(the contact layer) is formed. That is, the second electrodeis connected to a second portion of the second conductivity type (the semiconductor region) located on the opposite side to the substratewith respect to the light absorption layerin the semiconductor stacked portion. On the other hand, the first electrodeis connected to a first portionof the first conductivity type (a part exposed from the passivation filmin the buffer layer) located on the substrateside with respect to the light absorption layerin the semiconductor stacked portion.

is a sectional view taken along line IV-IV in. As illustrated in, a semiconductor stacked portionis formed on the front surfaceof the substratewith the buffer layerinterposed therebetween. The structure of the semiconductor stacked portionis the same as the structure of the semiconductor stacked portionother than the buffer layerexcept that the P-type semiconductor regionis not formed. The semiconductor stacked portionis covered with the passivation filmas a whole.

Here, the second electrodeextends from the top surfaceof the semiconductor stacked portionto a top surfaceof the semiconductor stacked portion(a surface on the opposite side to the substrate) and forms an anode padon the top surface. That is, the anode padelectrically connected to the second electrodeis formed on the top surfaceof the semiconductor stacked portionwith the passivation filminterposed therebetween.

is a sectional view taken along line V-V in. As illustrated in, semiconductor stacked portionsandare formed on the front surfaceof the substratewith the buffer layerinterposed therebetween. The structures of the semiconductor stacked portionsandare the same as the structure of the semiconductor stacked portionother than the buffer layerexcept that the P+-type semiconductor regionis not formed. The semiconductor stacked portionsandare covered with the passivation filmas a whole. Here, the first electrodeextends from a part bonded to the buffer layerto a top surfaceof the semiconductor stacked portion(a surface on the opposite side to the substrate) and forms a cathode padon the top surface

That is, the cathode padelectrically connected to the first electrodeis formed on the top surfaceof the semiconductor stacked portionwith the passivation filminterposed therebetween. On the other hand, a dummy padis formed on a top surfaceof the semiconductor stacked portionwith the passivation filminterposed therebetween. As illustrated in, the cathode pad(and the semiconductor stacked portion) is formed as a pair with the anode pad(and the semiconductor stacked portion) interposed therebetween, and a pair of dummy pads(and the semiconductor stacked portion) is also formed.

In the optical device A, the semiconductor light receiving elementis disposed such that the front surfaceof the substratefaces the sub-mount A, that is, such that the rear surfaceof the substratefaces the opposite side to the sub-mount A, and is mounted on the sub-mount A. Accordingly, a pair of cathode pads, the anode pad, and a pair of dummy padsare connected to the electrode pads provided on the sub-mount A. As a result, the cathode padand the anode padare connected to electrodes which are electrically connected to the trans-impedance amplifier Aon the sub-mount A.

As described above, the semiconductor light receiving elementhandles light in wavelength bands for optical communication such as 1.3 μm band, 1.55 μm band, and 1.6 μm band. In the semiconductor light receiving element, the light absorption layerprovided on the substrateof a semi-insulated semiconductor includes InGaAs. The In content x of the light absorption layeris equal to or less than (and less than 1). In this way, when the In content x of InGaAs in the light absorption layeris set to be equal to or greater than 0.55 (Graph Gin), the absorption coefficient is improved (improved to two times that in the 1.55 μm band in the example illustrated in), for example, in comparison with the case in which the In content x in Graph Gillustrated inis 0.53. Graph Ginindicates a case in which a light absorption layer formed of InGaAsP is used.

Accordingly, it is possible to curb a decrease in sensitivity even when the thickness of the light absorption layeris decreased to be equal to or less than 1.8 μm. That is, it is possible to achieve an increase in speed. In the semiconductor light receiving element, a particular structure (for example, the slope reflecting portion of the photodiode described in Patent Literature 1) does not need to be formed for the purpose of achievement an increase in speed. With the semiconductor light receiving element, it is possible to curb an increase in cost and to achieve an increase in speed. In view of an increase in speed, the semiconductor light receiving elementmay be configured such that an optical path oblique with respect to the thickness direction of the light absorption layeris formed in the light absorption layer.

As indicated by Graphs Gand Gin, an absorption end increases in wavelength by increasing the In content x of the light absorption layer. However, the semiconductor light receiving elementis characterized in that an absorption coefficient in the target wavelength band (the 1.3 μm band, the 1.55 μm band, and the 1.6 μm band for optical communication) is improved, not in that the absorption end increases in wavelength. That is, with the semiconductor light receiving element, it is possible to achieve a decrease in thickness of the light absorption layerand a high-speed operation by improving the absorption coefficient in the target wavelength band. In other words, in the semiconductor light receiving element, it is important to combine the target wavelength band and requirements of the In content x and the thickness of the light absorption layer. In addition, as the thickness of the light absorption layerdecreases, an influence of the absorption coefficient on a light sensitivity become more remarkable and thus the configuration for making a restraint response and a high light sensitivity compatible as in the semiconductor light receiving elementbecomes more effective.

On the other hand, when the In content x of the light absorption layeris changed (increased), a difference between the lattice constant of the light absorption layerand the lattice constant of the substrateis likely to increase and there is concern about crystallinity deterioration when the light absorption layergrows on the substrate.

Therefore, in the semiconductor light receiving element, the semiconductor stacked portionincludes the buffer layerstoserving as strain relaxation layers having a lattice constant between the lattice constant of the substrateand the lattice constant of the light absorption layer. Accordingly, it is possible to improve the crystallinity of the semiconductor stacked portionincluding the light absorption layer. Particularly, in the semiconductor light receiving element, the buffer layerstoserve as a plurality of strain relaxation layers which are disposed such that the lattice constant becomes close the lattice constant of the light absorption layerin a stepwise manner from the substrateto the light absorption layer. Accordingly, it is possible to reliably improve the crystallinity of the semiconductor stacked portion.

In the semiconductor light receiving element, the semiconductor stacked portionincludes the cap layerof the first conductivity type provided on the light absorption layeron the opposite side to the substratewith respect to the light absorption layerand including InAsP and the contact layerof the first conductivity type provided on the cap layeron the opposite side to the substratewith respect to the light absorption layerand including InGaAs, and the semiconductor regionof the second conductivity type is formed from the contact layerto the light absorption layervia the cap layer. The part connected to the second electrodeis the surface of the semiconductor regionformed in the contact layer. Accordingly, it is possible to decrease the contact resistance of the second electrode.

In the semiconductor light receiving element, the semiconductor stacked portionincludes the first semiconductor layer (the buffer layeror the buffer layer) of the first conductivity type disposed between the substrateand the light absorption layerand the second semiconductor layer of the first conductivity type (the buffer layer) with an impurity concentration lower than the impurity concentration of the first semiconductor layer disposed between the first semiconductor layer and the light absorption layer. Accordingly, it is possible to achieve a further increase in speed due to a decrease in capacitance.

In the semiconductor light receiving element, the substrateincludes a semi-insulated semiconductor. When a conductive substrate is used as the substrate, the substrate and the semiconductor stacked portionare electrically connected and thus have the same potential. In this case, capacitive coupling between an anode and a cathode is performed even via the passivation film(an insulating film) and thus a decrease in capacitance cannot be expected. On the other hand, in the semiconductor light receiving element, electrical isolation is possible by performing etching or the like of a grown layer to be the semiconductor stacked portionup to the semi-insulated or insulated substrate. As a result, it is possible to prevent capacitive coupling and achieve a decrease in capacitance. The substratecan be semi-insulated, for example, by doping InP, GaAs, or the like with Fe or the like. Since the lattice constant of InP matches the lattice constant of InGaAs, an InGaAs layer with good crystallinity can grow directly on the semi-insulated substrate.

The aforementioned embodiment is for describing an aspect of the present disclosure. Accordingly, the present disclosure is not limited to the aforementioned aspect and can be arbitrarily modified. Modified examples will be described below.

is a diagram illustrating a modified example of a method of mounting a semiconductor light receiving element. In an optical device B illustrated in (a) of, the semiconductor light receiving elementis mounted on a glass substrate Bsuch that the rear surfaceof the substratefaces the glass substrate Band is directly electrically connected to the trans-impedance amplifier Amounted on the glass substrate Bin the same way by a wire. The lens Aand the optical fiber Aare disposed on a surface of the glass substrate Bopposite to the surface on which the semiconductor light receiving elementis mounted, and light L is incident on the semiconductor light receiving elementfrom the rear surfacevia the glass substrate B. In an optical device C illustrated in (b) of, the lens Ais omitted in comparison with the optical device A illustrated in. The semiconductor light receiving elementmay be directly mounted on the trans-impedance amplifier A.